The present invention relates to method and apparatus for measuring the size of gas-suspended particles. Preferred embodiments of the invention relate to the measurement of the size distribution of carbonaceous particles such as soot or combustion products found in exhaust gases.
Measurement of gas-suspended particles is a subject of growing interest in the field of emission control. A convenient, accurate and inexpensive method for measuring emission particle size distribution is needed in the study of the effects and control of particulate emissions.
In the past, a number of different methods have been taught for the measurement of the size of gas-suspended particles. It has been proposed that particle size be measured by diffusion of light. Such a method is discussed in U.S. Pat. No. 3,825,345 to Lorenz, which notes disadvantages in this technique due to the complex functional relationship between particle size and the signal obtained, and the limitations of the technique due to an unfavorable signal to noise ratio for small particles. Somewhat similar techniques involving the detection of the effect of particles on light directed at them, rather than detecting light emitted by the particles, are taught in U.S. Pat. No. 3,680,961 to Rudd and U.S. Pat. No. 3,851,169 to Faxvog.
A second technique involves measurement of the thermal emission of particles by exposing the particles to a high temperature hydrogen flare. This method is also discussed in U.S. Pat. No. 3,825,345 to Lorenz where it is noted that, using this second technique, particles are singly and consecutively taken to a minute hydrogen flare in which they are evaporated. The particles are said to emit a luminescent flash of an intensity proportional to their mass. Lorenz indicates that this procedure is unsuitable for particle size analysis of atmospheric aerosols and often lacks sensitivity.
Lorenz proposes the use of an atomic absorption spectometer having an evaporating zone small enough to be completely filled by a single evaporated particle. However, in addition to the expense associated with the provision of an atomic absorption spectometer, the method is constrained to detecting particles, one at a time rather than detecting populations of gas suspended particles simultaneously.
U.S. Pat. No. 3,790,282 to Fielding teaches a method of determining the concentration of pollutants in air by exciting atoms of the pollutant by burning the pollutant with a fuel in a pressure chamber and measuring the intensity of the "characteristic light" emitted by the excited atoms. The Fielding patent indicates that from this intensity, the concentration of the pollutant can easily be calculated. The Fielding patent does not teach the measurement of size distribution of the pollutant particles.
It has been proposed to measure gas-suspended particle size distribution by the method of cascading inertial impactors. In such method a series of plates are positioned to intercept a gas stream carrying particles. Each plate is positioned behind an opening of a given size. The openings through successive plates are progressively smaller so that particles inertially adhering to each plate after impact are of different size ranges. After exposure of the plates to a gas-suspended particle stream for a given time, the plates are weighed. The measured weights are plotted against the plate opening size to indicate size distribution of the particles. The method has the disadvantages that it requires long measurement times and does not yield a continuous distribution.
Other art, of more general interest, is found in the following patents: U.S. Pat. No. 3,088,808 to Mandell, Jr.; U.S. Pat. No. 3,518,001 to Hell; U.S. Pat. No. 3,700,330 to Davis; U.S. Pat. No. 3,740,149 to Whetten; U.S. Pat. No. 3,860,345 to Raillere et al; U.S. Pat. No. 4,021,117 to Gohde et al; and U.S. Pat. No. 4,279,512 to Tunstall.
Accordingly, it is a primary object of the present invention to provide a novel, convenient, accurate and inexpensive method and apparatus for measuring gas-suspended particle size distribution.
It is another object of the present invention to provide a novel method and apparatus for measurement of the continuous size distribution of carbonaceous particles suspended in a gas such as an exhaust stream.
It is another object of the present invention to provide a novel method and apparatus to rapidly measure the size distribution of simultaneously detected populations of particles suspended in a gas.
These and other objects and features of the invention will become apparent from the claims, and from the following description when read in conjunction with the accompanying drawings.